SA Seasonal Rainfall Outlook: probabilities for October to December 2006,
issued 26th September 2006

Below average falls indicated for southern SA

There is a moderate shift in the odds towards drier than normal conditions for
the December quarter (October-December) over the agricultural areas of South Australia,
the Bureau of Meteorology announced today. Across the rest of the State,
the chances are generally close to 50% for accumulating at least median rainfall
during the coming three months.

The pattern of seasonal rainfall odds across SA is a result of higher than
average temperatures in both the Pacific and Indian Oceans, the former of which has been
warming strongly in the past few months.

Across southern SA, including southern parts of the Eyre Peninsula (see map), the
chances of the accumulated rainfall totals exceeding median rainfall
for the period October to December are between 30 and 40%.
Looking at the flip-side, these probabilities mean that BELOW
median falls have about a 60 to 70% chance of occurring.

So in years with ocean patterns like the current, about six or seven December quarters out of ten
are expected to be drier than average in these parts of the State, with about
three or four out of ten being wetter.

Outlook confidence is related to how consistently the Pacific and
Indian Oceans affect Australian rainfall. During the December quarter,
history shows this effect to be moderately consistent across the
eastern half of SA, but only weakly consistent in the west (see background information).

The Southern Oscillation Index (SOI), negative for the fourth straight
month, dropped from −9 in July to a low −16 in August.
The approximate SOI for the 30 days ending 19th September was −4.

In addition to the low SOI, the equatorial Pacific has been warming and
the Trade Winds have been weak. These ENSO indicators are all
consistent with the development phase of an El Niño event,
the likelihood of which has risen strongly in the past month.
For routine updates and comprehensive discussion on the latest data relating to
ENSO, together with details on what the phenomenon is and how it
has affected Australia in the past, please see the ENSO Wrap-Up.

Click on the map above for a larger version of the map. Use the reload/refresh button to ensure the latest forecast map is displayed.

More information on this outlook is available from 9.00am
to 5.00pm (CST) Monday to Friday by contacting the
Climate and Consultancy section in the Bureau's Adelaide
Office: (08) 8366 2664.

THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 26th OCTOBER 2006

Background Information

The Bureau's seasonal outlooks are general statements about the probability
or risk of wetter or drier than average weather over a three-month period.
The outlooks are based on the statistics of chance (the odds)
taken from Australian rainfall/temperatures
and sea surface temperature records for the tropical Pacific and Indian Oceans.
They are not, however, categorical predictions about future rainfall,
and they are not about rainfall within individual months
of the three-month outlook period.
The temperature outlooks are for the average maximum and minimum temperatures
for the entire three-month outlook period.
Information about whether individual days or weeks may be
unusually hot or cold, is unavailable.

Probability outlooks should not be used as if they were categorical forecasts.
More on probabilities is contained in the booklet
The Seasonal Climate Outlook - What it is and how to use it,
available from the National Climate Centre.
These outlooks should be used as a tool in risk management
and decision making.
The benefits accrue from long-term use, say over 10 years.
At any given time, the probabilities may seem inaccurate,
but taken over several years, the advantages of taking account
of the risks should outweigh the disadvantages.
For more information on the use of probabilities,
farmers could contact their local departments of agriculture or primary industry.

Model Consistency and Outlook Confidence:
Strong consistency means that tests of the model on historical
data show a high correlation between the most likely
outlook category (above/below median)
and the verifying observation (above/below median). In this
situation relatively high confidence can be placed in the
outlook probabilities.
Low consistency means the historical relationship, and
therefore outlook confidence, is weak.
In the places and seasons where the outlooks are most skilful,
the category of the eventual outcome (above or below median)
is consistent with the category favoured in the outlook
about 75% of the time.
In the least skilful areas,
the outlooks perform no better than random chance or guessing.
The rainfall outlooks perform best in eastern and
northern Australia between July and January,
but are less useful in autumn and in the west of the continent.
The skill at predicting seasonal maximum temperature peaks in
early winter and drops off marginally during the second half of the year.
The lowest point in skill occurs in early autumn.
The skill at predicting seasonal minimum temperature peaks in
late autumn and again in mid-spring.
There are also two distinct periods when the skill is lowest
- namely late summer and mid-winter.
However, it must always be remembered that the outlooks
are statements of chance or risk.
For example, if you were told there was a 50:50 chance
of a horse winning a race but it ran second,
the original assessment of a 50:50 chance could still have been correct.

The Southern Oscillation Index (SOI) is calculated using the
barometric pressure difference between Tahiti and Darwin.
The SOI is one indicator of the stage of El Niño or La Niña
events in the tropical Pacific Ocean.
It is best considered in conjunction with sea-surface temperatures,
which form the basis of the outlooks.
A moderate to strongly negative SOI (persistently below 10) is
usually characteristic of El Niño, which is often associated with below
average rainfall over eastern Australia, and a weaker than normal monsoon
in the north. A moderate to strongly positive SOI (persistently above +10)
is usually characteristic of La Niña, which is often associated with
above average rainfall over parts of tropical and eastern Australia,
and an earlier than normal start to the northern monsoon season.
The Australian impacts of 23 El Niño events since 1900 are summarized
on the Bureau's web site
(www.bom.gov.au/climate/enso/).